Valorization of shrimp waste into a thermally modified biosorbent for the removal of cationic and anionic dyes

Shrimp waste powder (SWP) and its mildly thermally treated derivative obtained at 200 °C in air (TT-SWP) were investigated as low-cost biosorbents for the removal of methylene blue (MB) and methyl orange (MO) from aqueous solution. The aim was to determine whether a simple reagent-free thermal treatment could improve the adsorption behavior of raw SW without harsh activation procedures. SWP and TT-SWP were characterized by SEM, EDX, FTIR, XRD, and ζ-potential analysis, and their adsorption performance was evaluated as a function of sorbent dose, pH, contact time, initial dye concentration, and temperature. Thermal treatment modified the surface morphology and near-surface properties of the material, producing a rougher and more fissured surface and improving adsorption behavior, particularly for MO. Adsorption was strongly pH-dependent, with MB favored at neutral to alkaline pH and MO under acidic conditions. Kinetic data were best described by the pseudo-second-order model, whereas equilibrium data were most consistently fitted by the Langmuir model, supporting a predominantly monolayer adsorption regime with limited heterogeneity. At 298 K, the Langmuir maximum adsorption capacities were 101.02 and 115.05 mg g−1 for SWP toward MB and MO, respectively, and 94.18 and 123.85 mg g−1 for TT-SWP. Thermodynamic analysis indicated endothermic MB adsorption and exothermic MO adsorption, with relatively low enthalpy values supporting adsorption dominated by physical interactions. Desorption and regeneration tests showed feasible reuse of both sorbents, although with progressive performance loss over repeated cycles. These findings support mild thermal treatment as a simple upgrading strategy for SW derived sorbents.